FR2839110A1 - UPRIGHT SYSTEM CONNECTING AN UNDERWATER FIXED TO A FLOATING SURFACE UNIT - Google Patents

Abstract

Riser system connecting a fixed underwater installation to a floating surface unit. It is of the type comprising at least one flexible pipe (10) arranged in a catenary and extending between the surface installation (2) and a buoy submerged (5), at least one riser (11) arranged in a catenary between said buoy (5) and the seabed (6), said buoy (5) being anchored on the seabed by means of a device anchor (9) comprising at least two tensioned anchor lines (7, 8), and it is characterized in that it comprises at least two mooring lines (18, 19) in catenary and on which are provided return means (12 to 14) which exert on said buoy (5) a return force which depends on the lateral movement of said buoy (5). Petroleum applications for exploitation at sea.

Description

emergent (7) on which a collector can slide.

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  DlL UPRIGHT SYSTEM CONNECTING A

  SOWS-MARINE INSTALLATION FIXED TO A UNIT OF

FLOATING SURFACE

  The present invention relates to a riser system (Riser Tower System in English) intended to connect a fixed underwater installation such as a wellhead or a manifold to a floating surface unit such as a platform or a FPSO type ship (Floating

Production Storage and Offloading).

  Exploitation of an offshore oil field becomes more and more complex as the water depths wind

  important which, nowadays, can reach several milkers of meters.

  o The transfer of product from the fixed installation, located on the seabed and constituted in particular by a wellhead, to the unit or floating surface installation, raises a certain number of difficulties. The most commonly used transfer systems are so-called riser systems or Riser Tower System, which include pipes in which circulate various products to be transported between the seabed and the surface, these products being for example oil, gas, water, etc. Other lines can also be used in particular fluid injection lines, charging or electrical and / or hydraulic control. o In petroleum operations, in particular for which deposits wind at great depths, the zones of turbulence which are located between 50 and 300 m below the surface of the water, can have effects not only on [ installation or surface unit which can undergo movements due to swell and other phenomena such as

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  pitch, roll, etc ... but also on the riser system

  which undergoes forces due to waves, wind and nostrils.

  As a result, riser systems are designed to resist

  to these requests which can be more or less important.

  Various types of riser systems have been proposed and described for example in US 3,111,692, 3,677,302, 4,031,919, 4,188,156,

  4 182 584, 4 38S, 022, 4 400 109 and 4 423 984.

  The main drawbacks of these systems lie in the fact that it is necessary to use buoys with high buoyancy of at least 2,000 tonnes when this buoyancy is distributed over the whole of the riser, the elements of the buoy have to endure significant pressures. Another drawback is that these systems are manufactured on land and must then be brought and installed on site, [all of these operations being delicate and expensive. In addition, it is very difficult to anchor the ends of the rigid sections on the seabed without using divers or very sophisticated engines such as ROV (Remote Operated Vehicle), which induces a significant cost of setting in place

  and monitoring during the exploitation of the deposit.

  In US 5,639,187, a system is described which combines rigid pipes and flexible pipes to achieve fluid communication between the fixed marine installation and the surface unit, the system comprising a submerged buoy which is anchored on the seabed by means of four coated anchor lines, each of the anchor lines being attached to the ends of the buoy and to the top of a sort of rectangle formed on the seabed, so as to minimize rotation buoy which could be caused by horizontal forces and the weight of the pipes extending between the buoy and the seabed. In fact, the system is of the type "legs of tension" ("tension leg" in English) which must withstand vertical loads of at least 1,500 tonnes and even more in order to resume, in particular during the installation, the buoyancy of the buoy which must be greater than the weight of the pipes in large numbers in such an operation. d It also has a buoyancy reserve to give the whole system the stiffness necessary to limit its lateral movements. ln effect, the lateral movements of s5 buoy are undesirable because they can broker rigid pipes at

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  spokes such that they can plastically deform the pipes and thus create an initiation of crushing by ovalization at the level of the inflection zone (Sag Bend in English). The inflection zone is located above the contact zone of the pipes on the seabed, said contact zone

  s usually called the Touch Down Point (TDP in English).

  The aim of the present invention is to propose at least one riser system or Riser Tower System which is more suitable for movements of the surface unit, simple to carry out and to set up and less expensive than the systems of the art. previous, with conditions

o comparable operations.

  An object of the present invention is a riser system intended to connect a fixed underwater installation to a surface unit, of the type comprising at least one flexible pipe arranged in catenary and extending between the surface installation and a submerged buoy, at least i5 a riser (Riser in English) arranged in catenary between said buoy and the seabed, said buoy being anchored on the seabed by means of an anchoring device comprising at least two lines of 'anchor "coated, and which is characterized in that it comprises at least two mooring lines in catenary and on which wind provided with o return means which exert on said buoy a restoring force which

  depends on the lateral movement of said buoy.

  An advantage of the present invention resides in the fact that, when the submerged buoy moves laterally, it is automatically returned to its initial or equilibrium position by the return cores whose force is variable, that is to say that they develop a strength of

  reminder which depends on the amplitude of the lateral movement of the buoy.

  According to another characteristic of the invention, the wind return means constituted by a ballast which is distributed on either side of the point or zone of contact of the mooring line on the seabed. The loosened part of the mooring line, located above the point of contact, in the direction of the buoy, mainly constitutes the restoring force. Such a structure is simple to carry out since the ballast can be of any kind such as chains, balls, weights or even pigs. In addition, it is easy to determine in advance the length of "recall" of the mooring line, ie the length of the loose part. Finally, it is possible to ballast a more

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  great length, than that which is necessary, of the portion of line resting on the seabed if one does not wish to anchor the end of the mooring line. In such a case, you must take care that the mooring line does not move too much and does not get tangled in the anchor lines

s stretched with the buoy.

  According to another characteristic, each mooring line is connected to the buoy by a bridle or goose paste which is placed below the buoy and which is intended to prevent or at least limit rotation

buoy.

  o Other advantages and characteristics will appear on reading the

  description of an embodiment of the present invention, as well as

  annexed drawings in which: FIG. 1 is a schematic elevation view of the system a

  nonstop column according to an embodiment of the invention.

  s Figure 2 is a schematic elevation view of the system

  column for several lateral positions of the buoy.

  The column system 1, shown diagrammatically in FIG. 1, is intended to connect a fixed underwater installation 2, constituted for example by a wellhead, a manifold or other collector and delivering a product coming from a petroleum tanker or other, to a floating surface unit or installation 3 such as a platform or an FPSO, the distance separating the surface installations 3 and submarine 2 being able to reach several milkers. At a certain distance from the surface 4 of the water and generally beyond the turbulence zone of the expanse s of water concerned, is immersed a buoy 5 which is anchored on the seabed 6 by two lines of anchor 7, 8 ("tether" in English) which tense between

  the buoy 5 and a dead body 9 or other anchor core (suction stack).

  One or more flexible pipes 10 extending in catenary between the surface unit 3 and the buoy 5 wind connected to one or more soontent pipes 11 (Riser in English), which extend in catenary between the buoy 5 and the installation fixed deep sea 2, so that a fluid communication is established between said installations 2 and 3. The rising pipes extending in catenary from the buoy to the narin bottom can be of any type such as rigid pipes commonly called SCR ( Steel Catenary ss Riser), with single or double jacket (Pipe in Pipe) and leads from pipes

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  flexible or hybrid pipes comprising at least one flexible part

and a rigid part.

  The riser system (Riser Tower System in English) comprises at least two mooring lines 18, 19 (mooring in English) arranged in catenary over the buoy 5 and the seabed 6. Each mooring line 18, 19 comprises in the lower portion 14 a portion 13 which is loosened by a ballast 12. This ballast 12 constitutes return means for the buoy, the return force depends mainly on the lateral movement which said buoy 5 could have and the cause of which may have as origin o a strong swell, sea currents and more generally the displacements of the surface unit 3. The remains 12 are distributed on each side of the contact zone 15 (Touch Down Point in English) which is the zone or the point , or - the mooring line joins the seabed 6. The remains 12 can be constituted by weights, balls, chains or even pigs. The wind remains distributed on both sides of the contact zone when the buoy

  is in its equilibrium position (middle position A in Figure 2).

  In position A which corresponds to that of FIG. 1, the mooring lines 7, 8 are substantially vertical and the lapped parts 13 of the mooring lines 18, 19 rest for the most part on the seabed 6. In position C, the injured parts 13 rest more on the seabed while in position B. the injured parts 13 are raised and thus develop a restoring force which tends to bring the system back to position A, the restoring force being variable according to the length lessee which is raised from the seabed by the movement of the - 2s corresponding mooring line, movement induced by the displacement

lateral of buoy 5 (figure 2).

  By way of example, the ballast of each mooring line 18, 19 is made up of

  tituated by 4 inch (10 cm) lines which are distributed over 100 m, knowing that when buoy 5 is at mid point (vertically), approximately where a third of the ballast is lifted from the ground and produces a tension of approximately 50 tonnes in each mooring line 18, 19. Obviously, these indications are given only by way of example, the choice and the arrangement of the elements constitute the ballast depending on the case under consideration. However, it is possible to indicate that the weight per unit of length which is set for

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  ballasting the catenary mooring line 18, 19 depends in particular on the

  distance between buoy 5 and seabed 6.

  Each tensioned anchor line 7, 8 is attached to a top 16 of a bridle or goose paste 17 which is fixed to the buoy at one end of this behind, and which is also intended to prevent or greatly limit the rotation of the buoy. The attachment points of the anchor lines 7, 8 on the flanges 17 preferably preferably substantially in the median plane passing through the longitudinal axis 18 of the buoy 5. In FIG. 1, the median plane comprising the anchor lines 7, 8 is materialized by

o dots X-X.

  Each mooring line 18, 19 is connected to one end 20 of the buoy 5 which is opposite to the other lateral end 21 to which is connected the riser or pipes ("Riser" in English). Wile can also be connected to the top 16 of the flange 17, so that the attachment points of the mooring lines 18, 19 are located substantially in the midline plane in which the attachment points to the buoy 5 of the lines are located.

7, 8.

  In a preferred embodiment of the invention, the buoy 5 is of variable buoyancy and it comprises several parts, for example trots or parts 22 to 24 each constituted by a hollow cylinder. Such a structure of the buoy 5 makes it possible to avoid having to develop very large forces at the level of the buoy 5, which forces depend in particular on the number and the weight of the risers which will be provided between the narin bottom 6 and the said buoy 5. Indeed, thanks to this compartmentalization of the buoy 5, each cylinder 22 to 24 constitutes a compartment which can be partially or totally defective as the risers are laid. Thus, in a first phase, the wind compartments are filled with an appropriate fluid such as water. Then, after the installation of the first riser, part of a compartment is vice and filled with gas, the empty quantity being a function of the weight of the riser installed. Then proceed sequentially and in the same way for the other risers. According to the described embodiment of the invention, the rising pipes 11 (Riser in English) are connected to the flexible pipes.

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  associated 10 by end connections in a manner known per se. These rising pipes 1 l wind supported by the buoy by a connection receptacle and suspension device materialized schematically on the fgure 1 and reference 30, in which their terminal end piece is housed. It should be noted that this device may include damping means intended to allow the risers a certain clearance.

  angular with respect to the buoy at their connection.

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Claims (10)

  1. riser system (1) intended to connect a fixed underwater installation (2) to a floating surface unit (3), of the type comprising o at least one flexible pipe (10) arranged in catenary and extending between the surface installation (2) and a submerged buoy (5), at least one riser (11) arranged in catenary between said buoy (5) and the seabed (6), said buoy (5) being anchored on the seabed by means of an anchoring device (9) comprising at least two tensioned anchoring lines (7, 8), characterized in that it comprises at least two mooring lines (18, 19 ) in catenary and on which wind provided with return means (12 to 14) which exert on said buoy (5) a force   return which depends on the lateral movement of said buoy (5).   2. riser system according to claim 1, characterized in that the wind return means constituted by at least one ballast (12)   arranged on the lower portion of each mooring line (18, 19).   3. riser system according to claim 2, characterized in that the ballast (12) is distributed on either side of the contact zone (15)   of the mooring line (18, 19) on the seabed (6).   5 4. Nonstop column system according to claim 3, characterized in that the part (13) of the ballast located above the contact zone (15) constitutes the restoring force.   5. riser system according to claims 1 and 2,   characterized in that each mooring line (18,19) is connected to the buoy so (5) by means of a paste (17) placed below the buoy (5),   said paste (17) preventing the rotation of said buoy (5).   6. nonstop column system according to claim 1, characterized in that the taut anchor lines (7, 8) are located substantially in the   nedian plane (X-X) passing through the longitudinal axis of said buoy (5). 9 283 911 0   7. riser system according to claim 2 or 5, characterized in that the buoy (5) is of variable buoyancy and consists of several parts (22 to 24).   8. riser system according to claim 2 or 5, characterized in that each mooring line (18, 19) is connected to one end (20) of the buoy (5) which is opposite to the other lateral end   (21) to which is connected the riser (11).   9. riser system according to claim 2 or 5, characterized in that each mooring line (18, 19) is connected to the buoy   o immersed in a point (16) located substantially in the median plane (X-X).   10. Method for producing a riser system (1) between an underwater installation (2) and a floating surface installation (3), which consists in immersing a buoy (5) which is connected to the surface installation by at least one flexible pipe (10) arranged in catenary and which is anchored on the seabed (6) by at least two tensioned anchor lines (7, 8), to have at least one riser pipe (11) in catenary between said buoy and the seabed, characterized in that it consists in exerting on the buoy (5) return means whose force depends on the lateral movement